The present invention relates to single-phase AC induction motors, and is more particularly concerned with a motor start circuit that controls the flow of AC current to the motor's auxiliary or start winding when the motor is started up.
At start up, AC single-phase induction motors require a starting mechanism to rotate the magnetic field of the field windings, so as to generate sufficient torque to start the rotor. The starting mechanism enables the rotor to overcome the static forces associated with accelerating the rotor and with the load imposed on it. Different motors require different amounts of additional torque at start up. Also, the amount of auxiliary current required depends on initial load conditions, and on the quality of the AC power.
The typical AC induction motor armature is equipped with two sets of windings, namely, one or more main or run windings for driving the motor at a normal operating speed, and an auxiliary or start winding to create the required starting torque. In order to provide the necessary rotating magnetic field for start-up, a phase shift capacitor is connected in series with the start winding. During start-up, both the run winding(s) and the auxiliary or start winding(s) are energized to bring the motor up to a sufficient operating speed. At that point, the start or auxiliary winding either drops out of circuit so that the motor operates on the run windings alone, or can be connected to a run capacitor but cut off from the start capacitor. In the event that a heavy load is encountered, and the motor rpm drops below its design operating speed, or stalls, the auxiliary winding can be cut back in as needed to increase motor torque, and overcome the increased load.
A control circuit or control device is employed to turn off the start current to the start winding once the motor has achieved a sufficient operating speed. This may involve a timer, a magnetic field sensor, or another arrangement that is sensitive to motor load or motor speed.
In a typical AC induction motor, the start capacitor is connected in series with a relay switch between the start winding and one of the main power conductors. This typically involves the relay normally closed (NC) contacts, so that power is applied immediately to the auxiliary winding at start up. A relay coil then pulls the relay switch open after operating speed is achieved to cut off the auxiliary current.
A problem can arise from intermittent application of power to the motor, i.e. to the main AC power conductors. If the power is switched on and off and back on, power to the relay coil will be intermittent, and this will cause the relay switch to chatter, i.e., to fluctuate between on and off. In a worst case, if the power is cut in and then out, the start capacitor will be left fully charged, i.e., ±165 volts in the case of 110 VAC (RMS). When the relay coil releases the switch, the NC contacts close, and the entire charge on the start capacitor will discharge through the relay contacts and then through the low-ohmage run and start windings. This can create a current of several hundred amperes for a short period of time, which must pass through the relay contacts, and can melt them and cause them to fuse. If the relay contacts are fused closed, the run windings will always have the full AC current applied, and can burn out. If the relay contacts are fused open, then the motor will not start.
Due to system rotary inertia, the rotor continues to spin after AC power is removed, and this spin can generate an emf in the run and start windings. Closure of the relay contacts at this time can impose a very high voltage on the start capacitor and can feed a high current through the relay contacts.
In the case of a pressurized load, such as a scroll compressor, there is a tendency for the rotor to spin backwards if power is suddenly removed. This will create another AC voltage source that can contribute to relay contact degradation.
It is a common practice to place a bleed resistor across the start capacitor to dissipate the charge on the capacitor between motor start operations. This bleed resistor typically has a value of about 16 kilohms, which is sufficient for normal operations. However, the discharge time for a large value start capacitor can be several seconds or more, and so the bleed capacitor will not protect the start circuit from capacitive discharge current in the event of a more rapid intermittent application of current to the motor.